There is a great need for the accurate detection of end-tidal CO.sub.2 with regard to the surgical environment. The production of end-tidal CO.sub.2 is indicative of how well a patient is obtaining necessary oxygen from inhaled air. The end-tidal CO.sub.2 is given off by the blood as it absorbs fresh oxygen in the lungs. If there is a decrease in the concentration of end-tidal CO.sub.2 in the exhalation breath of a patient during surgery, then steps must be taken by the surgical team to counter the end-tidal CO.sub.2 decrease. Commonly, a decrease in end-tidal CO.sub.2 means that there is too much anesthesia fluid going to the patient, essentially smothering the patient.
Infrared fluid detectors are currently available to detect end-tidal CO.sub.2. However, they lack the ability to be frequently and easily recalibrated to compensate for any instabilities in the respective detectors that may arise to yield a false reading. In addition, in the detectors where some type of calibration is preferred, the presence of a zero fluid and a span fluid are required. For instance, U.S. Pat. No. 4,205,913 to Ehrfeld, et al. describes a typical IR fluid detection apparatus which uses a filter heel disposed in the IR beam path to act as a chopper. The chopper causes a reference signal to pass as well as a signal proportional to the concentration of fluid being analyzed to pass. The IR radiation passing through a measuring cuvette is detected by an IR detector. A concentration value is subsequently computed.
U.S. Pat. No. 4,437,005 to Ophaff, et al. describes a nondispersive infrared fluid analyzer that utilizes thermal energy to expand the volume of a fluid. The expanded fluid whose increase corresponds to the concentration of reactive fluid in the volume, exerts a corresponding pressure on a diaphragm capacitor. The capacitor delivers an electrical measuring signal in proportion to the amount of movement on the diaphragm.
In U.S. Pat. No. 4,200,791 there is described a fluid analyzer using nondispersive infrared radiation. A rotary filter wheel positioned between an IR source and a fluid sample cell causes an IR detector to produce a pulsating output. The output is processed indicating the concentration of the constituents of the fluid mixture in the sample cell.
U.S. Pat. No. 3,180,984 to Fertig, et al. describes a stabilized comparison analyzer using radiation in the visible and infrared range. The radiant energy is produced from two sources of light energy and directed into a sample and comparison cell, wherein the radiation therefrom is converged onto a detector. A power oscillator circuit alternately energizes the sources to render the effect of alternately interrupting the beams. Initially, the amount of energy of each beam reaching the detector is equal. During analysis the measure of imbalance between the two beams is the signal produced from the detector and corresponds to the type of fluid being analyzed.
British Pat. No. 1,570,899 to Allred Chemical Corporation describes an infrared fluid analyzer. Therein infrared light is passed having a selective frequency range in the region of air absorption based for preselected molecular species. A secondary filter transmits light having a plurality of discrete frequencies which provides a detectable signal. The detectable material is transmitted through fluideous material whereby the intensity of the signal changes in proportion to the concentration of the preselected species.